If problems such as those typified in the field of invention are to be overcome, needle and syringe combinations must be inexpensive to produce, easy to operate and should conform to applicable standards in order that they be widely utilised in the avoidance of such problems.
UK Patent Application GB 2 266 667 A describes such a combination wherein a standard needle is coupled to a detachable end-piece which is affixed to a syringe barrel. A flanged plunger which runs within guides integral to the barrel may, upon termination of use, be engaged into said endpiece by means of rotation in the manner of a bayonet type fixing. Having achieved engagement between said plunger and endpiece the combination is withdrawn within the barrel of the syringe thus shielding the needle.
This system suffers from the fact that as the plunger may be repeatedly moved to the extremes of axial travel allowing both the intake and expulsion of fluids, an unavoidable volume (`dead-space`) of unexpelled fluid is inevitable since a further rotary motion is required in order to engage to the endpiece and there is also no means provided of evacuating fluid from within the central bore of the endpiece. It may be noted that such a dead-space is undesirable and, whilst this cannot be completely eliminated, it is preferred that this unexpelled volume be minimised.
Furthermore this syringe requires that both the plunger and endpiece are sealed to the barrel in order to eliminate the leakage of fluid. This is achieved by the use of separate compressible sealing components which are captured between the components to be sealed. Whilst the same compressible component is utilised twice to serve this purpose, there is undesirable additional cost associated with this solution.
Additionally it may be noted that this syringe utilises an annulus of perforations which are required to break or tear tangentially upon rotation of the plunger. This requires that the detachable portion of the endpiece and its support structure are formed integrally to each other as one component.
The device is such that tearing force occurs in one direction and in one plane. It has been observed that this single action does not provide the optimum method of
breaking the perforations or initiating the withdrawal of the plunger/endpiece/needle. Furthermore it is identified that components incorporating such break-off features are sometimes difficult and expensive to develop and may exhibit varying properties which may affect their performance.
Additionally, whilst the syringe provides a means of attaching the plunger/endpiece/needle combination to the barrel upon full retraction of the needle, there is little to inhibit the complete withdrawal of this combination from the barrel component, whereupon it would be re-exposed. In this eventuality the needle is once again undesirably exposed and/or subject to interference.
GB 2 266 667 A represents the closest prior proposal currently known to the applicant and the present invention was developed from it. Also known to the applicant, however, and disclosed herein for the record, are the following published patent specifications:
______________________________________ GB 1454540 GB 2048077 US 4790822 US 5098402 EP 0327061 EP 0402908 WO 92/05820 WO 92/09320 ______________________________________
Additionally, U.S. Pat. No. 5,242,419 discloses a syringe formed from a barrel having internal righthand threads at each end, a plunger which supports a piston near one end, and a tapered distal end provided with a lefthand thread. The needle has a cap with an internal lefthand thread and a flange configured to engage the internal threads of the barrel.
U.S. Pat. No. 4,675,005 is directed towards a disposable syringe having a locking device so that after use, a hub and cannula of the syringe can be detached from the cylindrical body, withdrawn thereinto, and locked thereto.